Patent GB-1117818-A (GB)
Title: Telephone switching systems
Inventor: HOHMANN LAWRENCE ANTHONY
IPC: H04Q3/00
Filed: 1965-10-06
Abstract: 1,117,818. Automatic exchange systems. WESTERN ELECTRIC CO. Inc. 6 Oct., 1965 [7 Oct., 1964], No. 42348/65. Heading H4K. In a PABX having both selective and direct call facilities, a register, taken into use for a call, initially starts a scanner that looks for the calling line in order to connect it to an allotted link circuit and finally in response to dialled digits sets the scanner to the position of the wanted line in order to connect that line to the link. Certain master subscribers have key sets which enable them to directly select other subscribers or exchange lines or to allow them to add on additional parties (conference), transfer calls or make interim calls. Other subscribers can gain access only to a master subscriber. Facilities for detecting permanent loop conditions are provided. Non-latching relays are used for the cross-point switches. The register is in effect merely a common control circuit for the switching array, it includes a dial tone and busy-tone source and a core array which acts as a free running device during the line finding operation and as a stepping device in response to dial pulses during a selecting operation. In the following, reference is made to a number of figures of the drawing, of which, however, only Figs. 3, 4, 10 and 11 are shown. The first digit or digits of a component reference identify the figure in which that component may be found. Internal dialled call.-A master station such as 3A initiates a call by lifting his handset and operating internal pick-up key PUKI. The consequent flow of loop current is then sufficient to trigger transistors 4Q1 and 4Q2 but insufficient to operate marginal relay 3SA. The resultant current on leads AN and 4SR acts as a start signal to an allotter (not shown) and the register (Figs. 9-11). The allotter marks lead L1R of a link circuit whereby relay 7L1R comes up so as to connect the link to the register. In the latter, the mark on 4SR brings up group relay 11T2 which pertains to the group of subscribers numbered 20-29 and which in operating connects the leads 0-9 from the scanner (Fig. 10) to the group of leads 20-29 connected to the selector leads e.g. 3A of the 20's group subscribers in Fig. 4. With the link connected to the register, a signal is supplied for triggering the various transistors and PNPN switches of the register control circuit so as to start the core scanner SSC (Fig. 10). The latter pulses switches 10Q21 &c. successively until the calling line is discovered by means of a continuous path extending through the switch e.g. 10Q21, lead 3A, resistor 4R2 and the loop to detector GSR. The control circuit responds to the operation of 10Q21 by applying battery to lead 121 in order to bring- up cross-point relay 3L1A. The scanning circuit restores to its home position. The operation of 3L1A is aided by capacitor 3C. Supervisory relay 3SA now comes up in order to apply holding ground to one side of 3L1A and to trigger transistors 7Q2 and 7Q1 over which holding battery is supplied. The register returns dial tone over lead 122 which is inductively coupled to the line wires via coil 7A. The subscriber then dials the one or two digits of the wanted party's number. A two digit number starts with a 2 or a 3 which digits are not used for subscribers identified by a single digit. If a 2 or 3 is dialled, relays 11T2 or 11T3 respond in order to connect the scanner leads to the relevant 10's group. The relays are controlled by transistors 11Q4 or 11Q5 connected to the scanner output leads. The scanner does not step in response to the first impulse of a train, this impulse being utilized to prepare the scanner so that it is in effect absorbed. However after the last impulse is received, a timing circuit delivers an additional pulse to cause the scanner to step on to its proper outlet whereat the switch e.g. 10Q21 breaks down in order to pulse the called party's crosspoint relay e.g. 5L139 of subscriber 39. If the called party is busy switch 10Q21 cannot trigger and the control circuit responds by reverting busy tone. Relay 5L139 locks up to ground on lead RDP1 and the register applies battery to lead 123 in order to bring up 7R1 in series with transistor 7QL1. 7R1 connects a ringing generator 801 to the common induction coil whereby both parties receive a ringing signal. The register now removes battery from lead 116 whereby 7L1R falls back and the allotter and register are released from the connection. In response to the called subscriber lifting his handset, his supervisory relay 5S39 comes up, 7R1 falls back to remove ringing, and the connection is established. Called party fails to answer.-If this is the case subscriber 3A can operate his conference key CNF whereby transistor 7Q3 switches on to bring-up relay 7DR1. The latter releases 5L139. On restoring CNF, 7Q3 turns-off, 7DR1 releases, 7QL1 turns-off and hence 7R1 falls back. If CNF is again depressed 7Q3 and 7DR1 come on again but this time initiate a request signal over lead RR1 to the allotter. The latter brings-up 7L1R so as to connect up the register which immediately reverts dial tone. Conference call.-With a connection set-up as described above, either party may bring an additional party into the call by operating his conference key. Since 7R1 is back at this time, the allotter is immediately signalled over RR1 whereby it brings up 7L1R to connect the link to the register. The latter reverts dial tone and the connection to a third, fourth or more parties is then completed as before. Interim or transfer call.-The master station may require to make an external call during the existence of his intra PBX call, in which case he operates his hold key H. As a consequence, transistor 3Q1 turns on and brings up 3H which inserts a holding impedance across the loop. He then closes one of keys PUKI to PUKn so as to gain access to an exchange line circuit. The exchange reverts dial tone and the master dials the required number. When the wanted party replies and assuming a triangular call is required, the master operates his add-on key A/O-1 whereby 3H falls back and an exchange line cross-point relay e.g. L1CO1 comes up to connect the exchange line via decoupling capacitors to the link. The master may withdraw from the call by hanging-up. The last PBX station to hang-up effects release of the connection. If the exchange line call is in progress first, the master may bring in a PBX subscriber by opening his hold switch, operating his PUKI key and dialling the extension as outlined above. When the latter replies, the add-on key is operated and a triangular or transfer call condition exists exactly as before. Operator recall, which may be required when a non-master is connected to an exchange line, is effected in the usual manner by actuating the switch-hook contacts. A transistorized circuit that includes Zener diodes for charging various capacitors (Fig. 7) responds to the flashing and causes a lamp and buzzer to operate in one of the master stations. The latter on recognizing the signal may then enter the connection by operating PUKI. If the substation had merely hung-up the charging circuits ensure that the link is released. Outward dialling by non-master.-The substation first gains access to a master (not described) who then seizes an exchange line as described above. The substation dials the wanted number after the master has withdrawn. A transistor 7Q10 in the switch-hook flashing circuit of Fig. 7 repeats the dialling impulses to a mercury type relay DP-1 in the exchange line circuit. Direct call.-A master station takes down his handset and operates the relevant direct call button DSS. As before an allotter seizes a free link to which the register becomes attached. With DSS operated, the closing of cross-point relay 3L1A causes transistor 6Q3 associated with the wanted station to switch-on. Consequently ground is applied, to selector lead SEL30 whereby cross-point 5L130 operates, and, to lead 110 which causes the register to apply operating potential to ringing relay 7R. If the wanted party is busy 6Q3 stays off and ground is applied to lead 109 alone whereby the register reverts busy tone. Impulse absorption.-If the digit 1 is dialledeither deliberately or due to accidental switchhook operation-transistor 9Q13, under the control of transistor 11Q6, is turned-off momentarily. Consequently PNPN switches 9Q2, 9Q12 and 9Q6 turn-off to prevent dial tone from being removed or ringing relay control battery from being applied. Permanent loop.-A circuit, which includes mono-stable pair 11Q20, 11Q21 and which controls PNPN switch 11Q19, is provided so that in the event that after 8 seconds from initiation of a call-as measured by capacitor 11C1, Zener diode 11D1 and resistor 11R1-no dialling impulses have been received, transistor 11Q21 turns-off to remove holding battery from lead 116. Consequently relay 7L1R in the link falls back so as to release the register. Miscellaneous.-The register includes circuits for distinguishing between ground applied to lead 124 for the purpose of recalling the register to a link (see above) and ground applied for removing busy tone in the event that a required third party in a conference or interim call tests busy. A time out circuit including capacitor 11C3 and Zener diode 11D3 is provided for releasing the link in the event that a calling subscriber hangs-up before dialling.